Optical encoders are typically employed as motion detectors in applications such as closed-loop feedback control in a motor control system. Many optical encoders are configured to translate rotary motion or linear motion into a two-channel digital output for position encoding.
Many optical encoders employ an LED as a light source. In transmissive encoders, the light is collimated into a parallel beam by means of a lens located over the LED. Opposite the emitter is a light detector that typically consists of photo-diode arrays and a signal processor. When a code scale such as a code wheel or code strip moves between the light emitter and light detector, the light beam is interrupted by a pattern of bars and spaces disposed on the code scale. Similarly, in reflective or imaging encoders, the lens over an LED focuses light onto the code scale. Light is either reflected or not reflected back to the lens disposed over the photo-detector. As the code scale moves, an alternating pattern of light and dark patterns corresponding to the bars and spaces falls upon the photodiodes. The photodiodes detect these patterns and corresponding outputs are processed by the signal processor to produce digital waveforms. Such encoder outputs are used to provide information about position, velocity and acceleration of a motor, by way of example.
Transmissive optical encoders typically generate code scale images having good contrast, and hence are capable of operating at high speeds with high resolution. The high contrast characteristic of most transmissive optical encoders also permits the outputs provided thereby to be easily interpolated to higher resolution. Transmissive optical encoders usually require that light emitters be placed opposite light detectors, and thus require a relatively high profile in respect of package design.
In reflective optical encoders, the light emitter and light detector often may be placed on the same substrate, and thus low profile designs, fewer materials and shorter assembly times may be realized. Reflective optical encoders typically suffer from low contrast, which in turn leads to low speeds and low resolution.
Imaging optical encoders feature many of the same advantages as reflective optical encoders, such as low profiles and cost, but also require diffusive code wheels. In addition, imaging optical encoders suffer from low diffusive reflectance and usually cannot operate at very high speeds.
Reflective optical encoders known in the art often suffer from several performance and application problems, such as stray light originating at the light emitter hitting the light detector directly, which can cause contrast degradation, lower encoder performance, and limit resolution. Known reflective optical encoders also typically comprise one encapsulated dome with an emitter-detector pair disposed therewithin, which often leads to poor light collimation and consequent limits on encoder performance and resolution. Known reflective encoders also typically feature limited encoding capability, such as a maximum of two channels of data encoding, or a single index channel.
Various patents containing subject matter relating directly or indirectly to the field of the present invention include but are not limited to, the following:
U.S. Pat. No. 4,451,731 to Leonard, May 29, 1984;
U.S. Pat. No. 7,182,248 to Foo et al., Jun. 10, 2008;
U.S. Pat. No. 7,385,178 to Ng et al., Nov. 11, 2008.
U.S. Pat. No. 7,400,269 to Wong et al., Jul. 15, 2008;
U.S. Pat. No. 7,394,061 to Saidan et al., Jul. 1, 2008;
U.S. Patent Publication No. 2006/0237540 to Saxena et al., Oct. 26, 2006, and
U.S. Pat. No. 2008/0024797 to Otsuka et al., Jan. 21 2008.
The dates of the foregoing publications may correspond to any one of priority dates, filing dates, publication dates and issue dates. Listing of the above patents and patent applications in this background section is not, and shall not be construed as, an admission by the applicants or their counsel that one or more publications from the above list constitutes prior art in respect of the applicant's various inventions. All printed publications and patents referenced herein are hereby incorporated by referenced herein, each in its respective entirety.
Upon having read and understood the Summary, Detailed Description and Claims set forth below, those skilled in the art will appreciate that at least some of the systems, devices, components and methods disclosed in the printed publications listed herein may be modified advantageously in accordance with the teachings of the various embodiments of the present invention.